1. Field of the Invention
The invention pertains generally to fuel management systems having an open loop calibration which includes provision for special condition calibrations and is more particularly directed to a special condition calibration for increasing the air/fuel ratio during decelerations.
2. Prior Art
Electronic fuel schedulers or electronic control units for regulating the air/fuel ratio of an internal combustion engine are conventional in the art. These schedulers provide, from a calculation or electronic computation based upon the operating parameters of the engine, an air/fuel ratio that is considered substantially ideal for the instantaneous conditions sensed.
The "best" air/fuel ratio at which the engine will operate under a given set of operational conditions is normally a tradeoff between the competing factors of driveability, emissions, and fuel economy. It is generally understood that richer air/fuel ratios are better for power and driveability, a substantially stoichiometric air/fuel ratio the most desirable for emissions, and lean air/fuel ratios the calibration that gives the best fuel economy. The schedule of desired air/fuel ratios for the electronic control unit can be derived from empirical tests of emissions, driveability, and economy tests and may include areas where the one criterion is more important than the others.
For example, under urban or in city driving, conditions emissions are considered of importance because of the congestion of automobiles present in a small area and the amount of pollutants at these slow speeds while at highway or freeway speeds, economy would be the overriding factor of consideration. In addition, for passing or accelerations and to ease starting and warm up situations, power and driveability must be factored into scheduling.
Any number of the various engine parameters may be sensed to calibrate the schedule of air/fuel ratios, but the most advantageous method is to measure mass air flow or mass fuel flow and calculate the other from the schedule.
An air/fuel controller having a calibration based upon the speed of the engine and the density of the air as a measurement of mass air flow has been successfully provided by a U.S. Pat. No. 3,734,068 issued to J. N. Reddy on May 22, 1973. The disclosure of Reddy is hereby expressly incorporated by reference herein. Reddy discloses a base calibration pulse width that is a function of the RPM of the engine and manifold absolute pressure. The duration of the pulse width is used to regulate fuel flow to the engine based upon a schedule. This base calibration is an open loop control of the air/fuel ratio as the operating parameters of the engine are sensed by the controller and a control signal which is the fuel pulse duration is developed therefrom.
If the air/fuel ratio schedule from which the control signal is calculated or the engine environment to which it is applied is different from the optimum design system, then the controller will not perform as required. The difference in engine environments are generally either because of manufacturing tolerances that change the response of the engine, or, as occurs with all mechanical devices, the ageing factor which is difficult to schedule.
It is known in the art that to solve many of the problems faced by open loop fuel schedulers a closed loop integral controller may be effectively utilized. The controllers are termed "closed loop" because they sense the result of an actual air/fuel ratio change and develop a control signal based therein rather than calculate an air/fuel ratio change from a desired schedule as does the open loop controller. One of the most advantageous of these controller systems is based upon the bi-level output of an exhaust gas composition sensor which indicates whether a rich or lean air/fuel ratio charge has been combusted by the engine. The controller incrementally leans the air/fuel ratio during a rich indication of the sensor and incrementally enriches the air/fuel ratio during a lean indication of the sensors, thereby causing the system to oscillate in a limit cycle about a desired air/fuel ratio. Illustrative of this type of controller is a U.S. Pat. No. 3,815,561 issued to Seitz which is commonly assigned with the present application. The disclosure of Seitz is hereby expressly incorporated by reference herein.
With the closed loop systems, a catalytic converter for reducing the remaining noxious components of exhaust gas is generally provided. The three major emission constituents that are minimized in a catalytic converter are oxides of carbon, hydrocarbons HC, CO, and oxides of nitrogen NOx. The conversion efficiency of the catalyst is, however, a direct function of the air/fuel ratio of the charge inducted into the internal combustion engine. For air/fuel ratios that are lean, the converter efficiency for the reduction of the NOx component is reduced drastically, while for rich air/fuel ratios the conversion of the CO and HC components falls off rapidly.
Only in a very narrow band around a stoichiometric air/fuel ratio where theoretically air and fuel should combine completely, according to their mass proportions, will there occur a substantially total conversion of all three emission components. The precision of the open loop scheduler with its closed loop correction maintains this air/fuel ratio within this operating band to provide the maximum conversion efficiency.
Above certain operating temperatures of the engine more of the NOx component of the exhaust emissions may form than can be processed by the converter even at maximum efficiency. At these temperatures, exhaust gas recirculation (EGR) is used to cool combustion temperatures in the cylinder below that region where high NOx generation will take place.
A common configuration for providing an exhaust gas recirculation path is to establish a conduit between the exhaust manifold and intake manifold which includes a control valve for regulating the amount of exhaust gas recirculated. The amount of exhaust gas recirculated will be a direct function of the difference in pressures between the intake and exhaust manifolds and by positioning the control valve in accordance with the intake manifold pressure, it has been found a relatively constant amount of EGR can be obtained. Usually, the control valve is positioned in this manner by a vacuum plenum which averages intake manifold pressure by expanding and contracting a flexible diaphragm attached to the valve.
This configuration, however, produces an inherent mechanical delay in the response of the EGR valve for changes in intake manifold pressure.
By fixing the exhaust gas recirculation to a constant percentage of the inducted air, its effect on the air/fuel ratio can be scheduled for in the open loop control portion of the electronic control unit. However, during transient conditions and mainly during decelerations when the absolute pressure in the intake manifold of the internal combustion engine drops rapidly, a fixed percentage of EGR is not maintained. Since the difference in pressures between the exhaust and intake manifolds increases, a greater than the scheduled percentage of EGR is inducted into the engine. The extra EGR dilutes the incoming air charge leaving it with less oxygen than the open loop scheduler has calculated there should be from its speed density measurements.
A rich air/fuel ratio is a consequence of the transient deceleration condition and causes a great enough air/fuel ratio shift to unbalance the catalytic converter and reduce its efficiency. This is termed a breakthrough of the air/fuel ratio band in which the catalyst is efficient. The significant loss of oxidation efficiency results primarily in high CO and HC emissions which continue until the EGR valve repositions itself correctly and the near stoichiometric scheduled air/fuel ratio is again produced.
The time required for the EGR system to vary from one operating point to another is a function of the time necessary for the exhaust back pressure to change and the time necessary for the EGR valve to respond to the pressure change. The response time of the EGR valve is the major component of this lag and is much slower than the response of the fuel scheduler. It would be, therefore, advantageous for the scheduler to sense this condition of the rich air/fuel ratio breakthrough and provide a lean out during this time period.